1. Field of the Invention
The present invention relates generally to an arrangement for accurately measuring the contour of a three dimensional surface such as a complex optical surface or the contour of a two dimensional surface or line, and more particularly pertains to a self-calibrating, absolute line or surface measuring system using a redundant arrangement of measuring points.
2. Discussion of the Prior Art
In greater detail, the subject invention is concerned in one particular aspect with the present state of the art in the development of computer controlled optical surfacing technology for producing complex shaped optical surfaces, such as infrared quality aspheric mirrors required for future generations of space telescopes, although the teachings herein have a much broader applicability as explained in greater detail below. The further development of this technology depends upon accurate measurements of the optical surface to provide data for surfacing machines which work, as by grinding, the complex optical surface.
In the context of this particular technological field, the development of a measuring arrangement to provide surface measurements as accurately as possible is an essential part of a program to develop a system for surfacing complex optical surfaces such as large aspherics. The measuring arrangement or machine should provide all of the surface data for the fabrication process, at least to the point at which quality interferograms can be produced. If the measuring machine is sufficiently accurate, then even the interferograms, which involve mirror specific special test optics and, in the case of very large mirrors, very large working distances, could possibly be eliminated. A high quality measuring machine could also reduce the measurement part of the fabrication cycle. The more accurate the measuring machine, the more accurate can be the surface grinding, and accordingly the polishing time can advantageously be reduced as a consequence thereof. In the context of this technology, therefore, the measuring machine is an important factor in minimizing the surfacing time, even if its accuracy is not sufficient for final figuring.
One known approach to measuring the contour of a convex or concave surface is to triangulate to a retroreflective target on the surface from a set of three observation points above the surface. In this known approach, triangulation from the three points depends upon an accurate measurement of the distances between the three points and the distances from the three points to the target on the surface. Unfortunately, it is frequently difficult to provide an accurate measurement of these distances.